__Question 5.5__:A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 ms

^{–1}. How long does the body take to stop?

__Solution__:*F*= –50 N

Mass of the body,

*m*= 20 kg

Initial velocity of the body,

*u*= 15 m/s

Final velocity of the body,

*v*= 0

Using Newton’s second law of motion, the acceleration (

*a*) produced in the body can be calculated as:

*F*=

*ma*

–50 = 20 ×

*a*

∴ a = -50/20 = -2.5 ms

^{-2}

Using the first equation of motion, the time (

*t*) taken by the body to come to rest can be calculated as:

*v*=

*u*+

*at*

∴ t = -u / a = -15 / -2.5 = 6 s

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